Such an optoelectronic safety device may, for example, be a closing edge safety device at a movable machine element, such as a gate or a gate wing, to monitor a danger zone, in particular a movement range of the machine element. It is thereby provided that the machine or gate movement is immediately stopped when the closing edge hits an obstacle. This is conventionally achieved by providing an electric circuit for the voltage supply to the safety device, in particular between the safety device and the machine control device which, as has previously been described, serves to control a drive motor of the movable machine element and to indicate an obstacle by transmitting the control signal or the motor release signal. Such a closing edge safety device has previously been described, for example, in EP 0 833 287 B1 and DE 10 2004 017 951 A1.
For monitoring the danger zone, optoelectronic safety devices have previously been described which comprise a light barrier with a light emitter and a light receiver forming a light signal path in the region of a moving edge of the machine element. Besides the optical coupling between the light emitter and the light receiver, an electric coupling is provided so that a feedback is actually provided between the light emitter and the light receiver. The light receiver sends a dynamic electric signal to the light emitter via an electric connection in response to the dynamic light signal emitted by the light emitter and received by the light receiver. This electric dynamic signal is most often detected by an evaluation unit, and a motor release signal is generated in response to the electric signal with the optoelectronic safety edges currently available on the market. Every failure in the light emitter/light receiver system will result in the absence of the electric dynamic coupling signal which is detected by the evaluating unit and is converted into a correspondingly negative motor release signal. In other words: the motor release signal is dependent on the optical and/or electric signal transmitted between the light emitter and the light receiver.
A line with three electric wires has become the standard for the electric connection to transmit the control signal and to realize an electric circuit for the operating voltage supply of the safety device, wherein typically two of the three wires are used to form the electric circuit and the third wire is used for signal transmission. It is thereby not only possible to provide a relatively undisturbed transmission of the control signal, the power supply to the safety device can also be effected.
In addition to this established type of connection, systems and manufacturers also exist which use an electric line with only two wires as a connection between the safety device and the machine control both for power supply and to transmit a control signal. The control signal and the supply voltage are frequently transmitted in a superposed condition with such connections.
Both of the above described systems require (depending on whether a two-wire or a three-wire connecting line is used) a safety device functional with and adapted to the respective system. A safety device with a three-pole or three wire terminal cannot be coupled with a machine control system having a two-wire terminal and vice versa. Such safety devices must therefore be specifically designed and manufactured for a system with a three-wire connection or for a system with a two-wire connection. Manufacturing and providing two separate safety systems that function almost identically and which merely have different transmission systems is, however, relatively complex and costly.